Views: 118 Author: Site Editor Publish Time: 2024-04-22 Origin: Site
Air flotation oil removal is the most commonly used and effective method for treating oily wastewater from refining and oil and gas field projects. This process involves introducing a large number of highly dispersed microbubbles into the sewage. These microbubbles act as carriers and adhere to the refined oil droplets and other suspended matter suspended in the water. They then float to the surface of the water relying on interfacial tension, bubble buoyancy, and other effects, where they are separated from the water.
There are different types of air flotation oil removal processes currently used in oily sewage treatment projects, including pressurized dissolved air flotation, vortex air flotation, induced air flotation, and electrolytic air flotation，impeller air flotation. It is essential to analyze and compare the characteristics of these different processes to determine their application and applicable occasions.
Pressurized dissolved air flotation is a water treatment process that uses a pressurized pump to mix oily sewage with air from an air compressor in a dissolved air tank. This creates dissolved air water with a certain pressure, which is then returned to the air flotation tank. The dissolved air water releases tiny bubbles under normal pressure, causing oil droplets to adhere to them and float to the surface, thus achieving oil-water separation.
This dissolved air method releases small bubbles with a diameter of generally 2
3\0-100 μm, with an average of 40 μm. In engineering applications, the process is divided into full dissolved air flotation and partial reflux dissolved air flotation. The gas source is usually air, but in treating oilfields, produced water with high salinity, nitrogen, or other fuel gases can be used instead of air to reduce equipment corrosion.
There is also a pressurized container air flotation type that uses a dissolved air pump to generate dissolved air water for air flotation separation. It replaces the dissolved air tank, pressurizing pump, and air compressor in the traditional pressurized dissolved air method.
Electric flotation technology is the electrolysis of wastewater to produce hydrogen and oxygen microbubbles that combine with pollutants in the water to float to the water surface for separation. The surface load of electric flotation is more significant than that of conventional air flotation, the hydraulic residence time is shorter, and the equipment size is small and compact. Still, the kinetic energy consumption and maintenance costs are higher. The process structure diagram is shown in the figure below.
The electrode plate in the electric float electrolyzes the incoming water, which generates bubbles. Simultaneously, nitric acid is added to break the emulsification. This process causes the oil dirt to float up and separate from the water, and then it is discharged. The resulting effluent is sent to the next unit for treatment.
Impeller air flotation equipment comprises a pool body, a gas-liquid mixer, a slag scraper, and a power control cabinet. The gas-liquid mixer is mainly used to provide the microbubbles required by the air flotation machine. It is a self-priming gas-liquid mixing equipment with an impeller. The impeller has a unique structure. The air enters the inside of the impeller relying on the negative pressure generated during the high-speed rotation of the impeller underwater, and is divided into micro-bubbles by the high-speed water flow in the supergravity centrifugal force field inside the impeller, and then collides with the stator mechanism outside the impeller twice, causing the air to enter the impeller. The microbubbles are further broken up and evenly dispersed in the water body.
Vortex concave air flotation is a process that uses a motor installed on the top part of the air flotation machine to rotate the submerged impeller at high speed. This creates a negative pressure inside the impeller, which sucks in air from the air inlet pipe and throws it out with the rotating impeller. The thrown-out air forms atomized bubbles, which are then thoroughly mixed with water under the impeller's stirring and then thrown out by the guide blades. These fine bubbles combine with oil droplets and flocs in the water, adhering to each other and quickly rising to the surface, resulting in oil-water separation.
The bubbles produced by vortex air flotation are larger in diameter, usually between 700 and 1500 μm. At the bottom of the vortex air flotation machine, there is a return pipe. When microbubbles are generated, a negative pressure area is formed at the bottom of the pool. This negative pressure causes the sewage to flow back from the bottom of the pool to the impeller aeration area and then return to the air floating section. This process ensures that about 40% of the sewage is returned, and the air flotation section can still work without water ingress.
Induced air flotation is a water treatment process that uses a circulation pump to pressurize a portion of the treated water, which is then returned to an ejector inside the air flotation machine. Through the principle of induction and injection, the return water flows through the ejector and draws in gas from the top of the air flotation machine. This gas is broken into bubbles ranging in size from 500 to 1000 micrometers by the ejector and thoroughly mixed with the treated sewage in the air flotation separation chamber. During the floating process, microbubbles adsorb suspended oil droplets to the liquid surface, achieving oil-water separation. The induced flotation machine operates without rotating parts, relying instead on hydraulic action to generate bubbles. In engineering applications, injectors may be installed inside or on top of the air flotation machine, with different injectors having a more significant impact on the size of the bubbles produced. Induced air flotation is widely used in the treatment of oilfield-produced water.
Pressurized dissolved air flotation is a process used for treating oily wastewater in the refining and chemical industry. The equipment used in this process includes the flotation tank body, releaser, slag scraper, mud discharger, return pump, air compressor, and dissolved air tank. The dissolved air pressure is typically designed to be between 0.4 and 0.5 MPa, with a reflux ratio of 30% to 50%, a gas-to-solid ratio of 0.02 to 0.10 kg[air]/kg[SS], and a surface hydraulic load of 1.2 to 2.0 L/(m2·s). The hydraulic retention time of the separation chamber is usually 40 to 60 minutes, and the adequate water depth of the flotation tank is generally 2.0 to 2.5 meters.
During electric floating operation, an appropriate amount of Na Cl must be added to the wastewater to ensure a greater current density under the same voltage, thus reducing the operating electricity cost. Since the cold rolling thick oil wastewater has a high conductivity (5000~ 6000μS/cm), there is no need to add additional metal salts. Take one #, two #, and three # wastewater into electric floats for treatment. The residence time is controlled at 30 minutes. The changes in the effluent quality with the incoming water pH (nitric acid is added to the incoming water to adjust the pH) are plotted.
When comparing a device with a sewage treatment scale of 100 m3/h, the impeller air flotation machine usually only requires one gas-liquid mixer to meet the requirements, with a power consumption of around 0.75 kW. The hydraulic retention time during the flotation process is approximately 20-30 minutes. The experimental object consists of six dissolved air flotation cells, with each cell having a size of 20.0 m x 4.5 m x 2.0 m and a total volume of 1080 m3, with return water accounting for around 50%-70% of the volume. By using the impeller air flotation method, a single tank can treat up to 300 m3/h of water.
The main control parameter for the impeller air flotation machine is the speed of the gas-liquid mixer. Its console can be installed indoors for centralized control.
The vortex concave air flotation equipment consists of the flotation tank body, vortex concave aerator, slag scraper, and slag discharge device. The impeller diameter, rotation speed, and suction pipe installation position are the key design factors of vortex concave air flotation. The impeller diameter should generally be between 200 to 400 mm, and should not exceed 600 mm at maximum. The impeller speed should range from 900 to 1500 r/min, with a circumferential linear speed of 10 to 15 m/s. The air flotation separation time is usually between 20 to 25 minutes. The water depth of the air flotation tank should generally be between 2.0 to 2.5 m, and must not exceed 3.0 m. A single air flotation unit should be square, with a single side size not greater than six times the diameter of the impeller. The impeller requires a power of approximately 1.8 kW/m3.
Induced air flotation is a process that involves equipment like the air flotation tank, injectors, and circulation pumps. The equipment is designed as a horizontal cylindrical structure, which consists of four air flotation separation chambers in series, a water outlet unit, and a scum collection unit. Each separation chamber unit has a set of injectors that work by absorbing water from the water outlet unit and pressurizing it to send it as a working fluid to the ejector. The water then ejects through the ejector and inhales gas, releasing bubbles in the separation chamber to complete the air flotation separation process. The duration of the air flotation separation time is usually between 4 to 8 minutes, and the residence time of a single separation chamber is about 1 to 2 minutes. For specific oil products, the air flotation separation time can be between 8 to 20 minutes, and the reflux ratio usually ranges from 20% to 30%.
Pressurized dissolved air flotation is a highly efficient method used to treat oily wastewater in industries like oil refining and chemical production. The system produces stable and fine bubbles, ensuring an ideal air flotation effect. It can adapt to varying incoming water loads, thanks to the circulation pump, which can regulate the amount of dissolved air and water. However, operating the system is complex due to the pressure and liquid level control of the dissolved air tank, as well as the operation control of the reflux pump and air compressor. Shutting down and restarting the system is also cumbersome. System maintenance is complicated, and the nursing workload is heavy.
Moreover, the operating costs are relatively high due to the required reflux pump and air compressor. Assuming the return pump head is 30-60 m, the air compressor pressure is 0.4-0.5 MPa G, and the electricity cost is 0.8 yuan/(kW·h), the energy consumption per ton of water in the aeration system alone is about 0.8-1.6 yuan. Several petrochemical and oil refineries use pressurized dissolved air flotation to treat their wastewater. For instance, a petrochemical refinery's 950 t/h sewage treatment plant uses the flotation unit with a mass concentration of incoming water oil at 100 mg/L and an average mass concentration of effluent oil at 20 mg/L. Similarly, a petrochemical sewage treatment plant and a sewage treatment plant of the Second Petroleum Plant of a petrochemical company use pressurized dissolved air flotation. The former has a mass concentration of incoming oil at 68.37 mg/L, and the effluent is 50.22 mg/L. The latter treats 1500 t/h of water with a mass concentration of incoming water oil at 50-80 mg/L and produces effluent water at 11-15 mg/L. Also, a refinery in the northeast of PetroChina built a new sewage treatment plant in 2007 and uses two sets of inclined plates to pressurize dissolved air flotation in its oil removal unit. The processing capacity of a single set of equipment is 175m3/h, and the designed mass concentration of oil in the air flotation unit is 100 mg/L, with the mass concentration of the effluent being less than 10 mg/L.
The study involved testing the effluent from the regulating tank under different conditions and periods. The oil content in the effluent ranged from 1500 to 40000 mg/L, while the SS was between 500 and 9000 mg/L. The oil content was mostly around 6000 mg/L, while the SS was around 4000 mg/L. Three types of wastewater were tested, labeled 1#, 2#, and 3#. The first type contained oil 40000 mg/L and SS 9000 mg/L, the second type contained oil 6000 mg/L and SS 4000 mg/L, and the third type contained oil 2500 mg/L and SS 1200 mg/L. The pH changes were monitored by adding nitric acid to the incoming water to adjust the pH, and the results are shown in Figure 1 below. The treatment effects are summarized in Table 1 below.
| Table 1 Optimum pH treatment effect of electrical flotation | |||||
| Project | Best water outlet(PH) | Oil content(mg/L) | Removal rate/% | SS(mg/L) | Removal rate/% |
| 1#Wastewater | 6.5 | 2100 | 94.8 | 240 | 97.3 |
| 2#Wastewater | 6.5 | 750 | 87.5 | 161 | 95.8 |
| 3#Wastewater | 7.0 | 340 | 86.4 | 76 | 93.6 |
It can be seen from Figure 1 that the removal rate of oil and SS in the electroflotation inlet water decreases as the inlet water concentration decreases, and the effluent water quality increases as the inlet water concentration decreases. Its treatment effect under acidic inlet water conditions is better than that of alkali. Due to the permanent water inflow, comprehensive consideration of the cost of dosing, it is appropriate to control the pH of the electrical float inlet water to 6.5~7.0 in actual operation. The operating current of the electrical flotation equipment is adjusted as the inlet water concentration changes to ensure the quality of its outlet water. The curve of the operating current changing with the inlet water concentration is shown in Figure 2 (the inlet water pH is controlled to 6.5, and the residence time is controlled at 30 min,).
figure 2
The quality of water in the electric float is observed to increase with the increase of the operating current in the three water inlet conditions. In order to ensure a good quality of outlet water, a higher operating current is required for water with a high concentration. However, if the inlet water concentration becomes too high and the operating current is still controlled at a low value, it may lead to a decline in the quality of produced water and cause blockage in the subsequent ultrafiltration system.
The effect of the amount of flocculant added on the oil content of the wastewater was studied through two tests - a beaker test and a continuous operation test. It was determined that the ideal dosage of flocculant (per unit volume of wastewater) is 180 mL/m3 (21 mg/L) of poly aluminum, 180 mL/m3 (21 mg/L) of poly aluminum, and 180 mL/m3 (2.1 mg/L) of Acrylamide. This dosage results in stable water quality in the air flotation machine.
When polymerized aluminum is added alone, the scum formed is less dense and the effluent carries more fine suspended matter. However, when both polymerized aluminum and polyacrylamide are added simultaneously, the scum formed is denser and the effluent is clearer, carrying fewer suspended solids. Therefore, it is recommended to add both flocculants simultaneously during the stable operation of the air flotation machine.
Furthermore, the dosage of flocculant in the workshop flotation tank should be 169 mL/m3 (20 mg/L) of poly aluminum and 131 mL/m3 (1.6 mg/L) of polyacrylamide.
The changes in oil content in the inlet and outlet water of the impeller air flotation machine and workshop flotation tank are shown in Figure 3.
Figure 3
It can be seen from Figure 3 that under normal incoming water conditions with good water quality, the oil content of the water produced by the impeller air flotation machine is basically close to that of the water produced by the flotation tank in the workshop; when the incoming water is black, has high oil content and suspended matter content, , the water quality of the impeller air flotation machine also remained basically stable, but the oil content of the water was lower than that of the workshop flotation tank. The test results show that under normal circumstances, the impeller air flotation machine can achieve or be better than the oil removal effect of dissolved air flotation.
The main component of the vortex concave air flotation is the vortex concave aerator. It has few ancillary equipment and can be used immediately, so the operation and maintenance workload is small. The main energy-consuming component is the vortex concave aerator, which has low energy consumption. The water treatment capacity of a single aerator with a shaft length of 2m is about 30~150m3/h, and the power is 2.2kW. The corresponding water and electricity consumption per ton of the aeration system is 0.01~0.06 yuan.
Vortex air flotation has a unique way of dissolving air. The floc is formed only after aeration, otherwise it will easily lead to the destruction of the floc. Therefore, it is also highly dependent on the selection of coagulation and flocculation agents. At the same time, the adaptability of vortex concave air flotation to water inlet impact load is also small, and changes in water inlet load have a greater impact on the treatment effect. Due to its simple equipment, convenient operation, and energy saving, a petrochemical sewage treatment plant adopted a two-stage vortex air flotation treatment process after renovation. The average mass concentration of oil in the influent water of the first-stage vortex air flotation was 190 mg/L. , the average mass concentration of oil in the water floated by the secondary vortex air floatation is 25 mg/L, and the removal rate is 86%. In 2004, a petrochemical refinery upgraded the second sewage treatment plant. The air flotation unit was transformed from the original dissolved air flotation to a vortex air flotation. The design treatment capacity was 600 m3/h. The mass concentration of water and oil in the vortex air flotation was When it is 80~150 mg/L, the mass concentration of the effluent oil is 10~20 mg/L. In 2003, part of the dissolved air flotation in the sewage treatment unit of Sinopec Cangzhou Refinery was modified to use vortex concave air flotation. The average mass concentration of oil in the inlet water of vortex concave air flotation was 118 mg/L, the effluent was 16 mg/L, and the removal rate reached 86%. . The supporting sewage treatment plant of a branch of Sinopec uses three vortex concave air flotation systems with a scale of 320 t/h as the pretreatment process for biochemical treatment. The average mass concentration of oil in the incoming water in summer is 215.23 mg/L, and the average mass concentration of oil in the effluent water is 215.23 mg/L. The concentration is 54.71 mg/L, the average mass concentration of oil in the inlet water in winter is 135.34 mg/L, and the average mass concentration of oil in the outlet water is 49.76 mg/L.
Unlike dissolved air flotation, induced air flotation does not require dissolved air equipment (dissolved air tank and air compressor), has low energy consumption, and is more straightforward to operate and maintain. and lure
Air flotation is more straightforward to operate in a closed system. For example, in treating produced water with high salinity, using nitrogen or fuel gas as the gas source can avoid contact with oxygen in the air and prevent the sewage from being treated. Harmful gases are released into the external environment, causing pollution. The amount of dissolved gas in return water can be adjusted through the circulation pump, which has greater adaptability to the incoming water load. The only energy-consuming equipment in the aeration system is the circulation pump. Based on a return ratio of 30% and a lift of 20 to 40 m, the energy consumption per ton of water is approximately 0.03 to 0.04 yuan.
Because the system design has good airtightness, induced air flotation is widely used in oilfield-produced water treatment, especially in foreign countries. Daqing Oilfield Xingshilian Oily Wastewater Station introduced a set of induced air flotation and micro-dynamic filtration processes from abroad to treat oily wastewater. The test treatment scale was 4 000 m3/d, the reflux ratio was 40%, and the mass concentration of oil in the air flotation inlet water was 100.31 mg/L, and the effluent was 29.53 mg/L[16]. Veolia uses induced air flotation to treat oil field-produced water in two Jachfish projects of Devon Energy Group in Canada. The treatment scale is 932 m3/h. The mass concentration of oil in the air flotation designed inlet water is 100 mg/L, and the effluent water is ten mg/L.. Mobil Oil Company uses induced air flotation to treat oily wastewater at the ImperialOil Cold Lake fifth-produced water treatment plant, using fuel gas as the dissolved gas medium. The single treatment scale is 995 m3/h, the return flow is 249 m3/h, and the air flotation The average mass concentration of oil at import is 200 mg/L and at export is 10 mg/L.
